In rechargeable alkaline cells, weight and portability are important considerations. It is also advantageous for rechargeable alkaline cells to have long operating lives without the necessity of periodic maintenance. Rechargeable alkaline cells are used in numerous consumer devices such as portable computers, video cameras, and cellular phones. They are often configured into a sealed power pack that is designed as an integral part of a specific device. Rechargeable alkaline cells can also be configured as larger cells that can be used, for example, in industrial, aerospace, and electric vehicle applications.
The materials proposed in the prior art for use as hydrogen storage negative electrode materials for secondary batteries are materials that have essentially simple crystalline structures. In simple crystalline materials, limited numbers of catalytic site are available resulting from accidently occurring, surface irregularities which interrupt the periodicity of the crystalline lattice. A few examples of such surface irregularities are dislocation sites, crystal steps, surface impurities and foreign absorbates. For more than three decades, virtually every battery manufacturer in the world pursued such crystalline electrode materials for electrochemical applications, but none produced a commercially viable nickel metal hydride battery until after the publication of U.S. Pat. No. 4,623,597 (the '597 patent) to Ovshinsky, et al, which disclosed fundamentally new principles of electrode material design.
As taught in the '597 patent (the contents of which are incorporated by reference), a major shortcoming of basing negative electrode materials on simple ordered crystalline structures is that irregularities which result in the aforementioned catalytically active sites occur relatively infrequently. This results in a relatively low density of catalytic and/or storage sites and consequently poor stability. Of equal importance is that the type of catalytically active sites available are of an accidental nature, relatively few in number and are not designed into the material as are those of the present invention. Thus, the efficiency of the material in storing hydrogen and its subsequent release is substantially less than that which would be possible if a greater number and variety of sites were available,
Ovshinsky, et al, fundamental principles overcome the limitations of the prior art by improving the characteristics of the negative electrode through the use of disordered materials to greatly increase the reversible hydrogen storage characteristics required for efficient and economical battery applications. By applying the principles of disorder, it has become possible to obtain a high energy storage, efficiently reversible, and high electrically efficient battery in which the negative electrode material resists structural change and poisoning, with improved resistance to the alkaline environment, good self-discharge characteristics and long cycle life and deep discharge capabilities. The resulting disordered negative electrode materials are formed from lightweight, low cost elements by techniques that assure formation of primarily non-equilibrium metastable phases resulting in high energy and power densities at low cost. These non-equilibrium, metastable phases assure the formation of localized states where a special degree of disorder, if properly fabricated, can come from the structural and compositional disorder of the material.
The materials described generally in the '597 patent have a greatly increased density of catalytically active sites providing for the fast and stable storage and release of hydrogen. This significantly improved the electrochemical charging/discharging efficiencies and also showed an increase in hydrogen storage capacity. Generally, this was accomplished by the bulk storage of hydrogen atoms at bonding strengths within the range of reversible electromotive force suitable for use in secondary battery applications. More specifically, such negative electrode materials were fabricated by manipulating the local chemical order and hence the local structural order by the incorporation of selected modifier elements into the host matrix to create the desired disorder, type of local order and metal hydrogen bond strengths. The resulting multicomponent disordered material had a structure that was amorphous, microcrystalline, multiphase polycrystalline (but lacking long range compositional order), or a mixture of any combination of these structures.
The host matrix of the materials described in the '597 patent were formed from elements capable of storing hydrogen an thus are considered hydride formers. This host matrix was modified by incorporating selected modifier elements which could also be hydride formers. These modifiers enhanced the disorder of the final material, thus creating a much greater number and spectrum of catalytically active sites with an increase in the number of hydrogen storage sites. Multiorbital modifiers (such as transition elements) provided the greatly increased number of sites due to various bonding configurations available. Because of more efficient storage and release of hydrogen and because of the higher density of the catalytic sites, the hydrogen more readily found a storage site. Unfortunately, there remained, until U.S. Pat. No. 5,840,440 ('440), an insufficient density of new hydrogen storage sites formed due to disorder to significantly increase the hydrogen storage capacity of the material.
The '597 patent describes the use of, inter alia, rapid quench to form disordered materials having unusual electronic configurations, which results from varying the three-dimensional interactions of constituent atoms and their various orbitals. Thus, it was taught that the compositional, positional and translational relationships of the constituent atoms were not limited by crystalline symmetry in their freedom to interact. Selected elements could be utilized to further control the disorder of the material by their interaction with orbitals so as to create the desired local internal chemical environments. These various and at least partially unusual configurations generate a large number of catalytically active sites and hydrogen storage sites not only on the surface but throughout the bulk of the material. The internal topology generated by these various configurations allowed for selective diffusion of hydrogen atoms.
In general, disorder in the modified material can be of an atomic nature in the form of compositional or configurational disorder provided throughout the bulk of the material or in numerous regions or phases of the material. Disorder can also be introduced into the host matrix by creating microscopic phases within the material which mimic the compositional or configurational disorder at the atomic level by virtue of the relationship of one phase to another. For example, disordered materials can be created by introducing microscopic regions or phases of a different kind or kinds of crystalline phases, or by introducing regions of an amorphous phase or phases, or by introducing regions of an amorphous phase or phases in addition to regions of a crystalline phase or phases. The types of disordered structures that provide local structural chemical environments for improved hydrogen storage characteristics include amorphous materials, microcrystalline materials, multicomponent multiphase polycrystalline materials lacking long range composition order or multiphase materials containing both amorphous and crystalline phases.
Short-range, or local, order is elaborated on in U.S. Pat. No. 4,520,039 to Ovshinsky, entitled Compositionally Varied Materials and Method for Synthesizing the Materials, the contents of which are incorporated by reference. This patent discloses that disordered materials do not require periodic local order and how spatial and orientational placement of similar or dissimilar atoms or groups of atoms is possible with such increased precision and control of the local configurations that it is possible to produce qualitatively new phenomena. In addition, this patent discusses that the atoms used need not be restricted to "d band" or "f band" atoms, but can be any atom in which the controlled aspects of the interaction with the local environment and/or orbital overlap plays a significant role physically, electronically, or chemically so as to affect physical properties and hence the functions of the materials. The elements of these materials offer a variety of bonding possibilities due to the multidirectionality of f-orbitals, d-orbitals or lone pair electrons. The multidirectionality ("porcupine effect") of d-orbitals provides for a tremendous increase in density of sites, the spectrum of types of sites and hence the presence of active storage sites. Following the teaching can result in a means of synthesizing new materials which are disordered in several different senses simultaneously.
The '597 patent is described in detail above because this patent represents a starting point for the investigation that resulted in the present invention. That patent introduced the concept of making negative electrode material for nickel metal hydride batteries from multicomponent disordered alloys. This teaching was diametrically opposed to the conventional "wisdom" of battery manufacturers at the time. It was not until this concept was adopted in production processes by said manufacturers that negative electrode materials with an increased number of catalytically active sites were produced and nickel metal hydride batteries became commercially viable. In capsule form, the '597 patent taught that significant additional sites for hydrogen catalysis (to allow the rapid storage and release of hydrogen and greatly improve stability) were formed and made available by purposely fabricating disordered negative electrode material (as opposed to the homogeneous, ordered polycrystalline material of the prior art). The '597 patent also proposed that the use of disorder could be employed to obtain additional hydrogen storage sites. However, it was not appreciated that in order to obtain a substantial increase in hydrogen storage capacity from such non-conventional storage sites, it would be necessary to increase the number of storage sites by approximately 3 orders of magnitude.
Not only was the teaching of the '597 patent adopted by all nickel metal hydride manufacturers, but in recent years some of these manufacturers have begun to use rapid solidification techniques (as taught by Ovshinsky) to increase the degree of disorder within a negative electrode alloy formula. For instance, battery companies have even gone so far as to rapidly quench highly-modified LaNi.sub.5 -type electrochemical negative electrode material. By employing nonequilibrium processing techniques, the resulting negative electrode material includes hydrogen storage phases and catalytic phases on the order of 2000 Angstroms in average dimension. The hydrogen storage capacity of the negative electrode material does not improve significantly, but the catalytic activity is greatly improved as evidenced by improved rate capability and stability to oxidation and corrosion, resulting in increased cycle life.
As mentioned above, certain battery companies have begun to investigate the use of rapidly-quenched, highly modified LaNi.sub.5 type hydrogen storage materials for electrochemical applications. For example, in Phys. Chem 96 (1992) No. 5 pp. 656-667, P. H. L. Notten, et al of Philips Research Laboratories presented a paper entitled "Melt-Spinning of ABS.sub.5.5 -Type Hydride Forming Compounds and the Influence of Annealing on Electrochemical and Crystallographic Properties." In this paper, non-stoichiometric modified LaNi.sub.5.5 materials, La.sub.6 Nd.sub.2 Ni.sub.3 Co.sub.24 Si.sub.1 and La.sub.6 Nd.sub.2 Ni.sub.26 Co.sub.24 Mo.sub.1 were rapidly solidified. These non-stoichiometric materials were compared to their stoichiometric counterparts with the result being that the non-stoichiometric materials demonstrated good, but not unusual hydrogen storage capacity. However, the non-stoichiometric compounds did show the presence of additional volume percents of a catalytic phase, which phase, as predicted by the '597 patent, was responsible for the improved electrochemical properties as compared to the properties found in the examples of stoichiometric material. Once again, and more importantly, no significantly higher density of non-conventional hydrogen storage sites were obtained. In research and development activities at Energy Conversion Devices, Inc. with highly modified TiNi-type electrochemical negative electrode materials, such as described in U.S. Pat. No. '440 which is incorporated herein by references, rapidly quenched electrode materials were melt spun. The work resulted in improved oxidation and corrosion resistance and cycle life was increased by a factor of five. On the other hand and as was true in the case of the aforementioned Japanese work, no significant increase in hydrogen storage capacity was demonstrated and the phases of the negative electrode material present were also relatively large.
Therefore, while the teachings of the '597 patent were revolutionary for those of ordinary skill in the art in learning to apply the principals of disorder disclosed therein to negative electrode materials to obtain commercial batteries with commercially viable discharge rates and cycle life stabilities while maintaining good hydrogen storage capacity, the '597 patent provided for the most part generalities to routineers concerning specific processes, processing techniques, alloy compositions, stoichiometries in those compositions, etc. regarding how to further significantly increase the hydrogen storage capacity (as opposed to the catalytic activity). It was not until the '440 patent that a teaching was presented of the nature and quantification of additional active storage sites required to significantly increase the hydrogen storage capacity of the negative electrode material through the deliberate introduction of defect sites and the presence of other concurrent non-conventional and/or conventional storage sites.
Despite the exceptional electrochemical performance now provided by current highly disordered nickel metal hydride systems (twice the hydrogen storage capacity of conventional NiCd systems) consumers are demanding increasingly greater run times, safety and power requirements from such rechargeable battery systems. No current battery system can meet these demands. Accordingly, there exists a need for a safe ultra high capacity, high charge retention, high power delivery, long cycle life, reasonably priced rechargeable battery system.
While U.S. Pat. No. 5,840,440 ("the '440patent") represents innovative ideas with respect to useable storage sites in an electrochemical negative electrode material due to the use of high defect density and small crystallite size, the focus of the '440 patent is on the bulk properties of the hydrogen storage alloy. Significant discussion therein relates to increased surface sites; however, the additional sites so described relate to the interior surfaces, or grain boundaries, again within the alloy. The '440 patent does not address the interface between the metal hydride alloy and the electrolyte at the so-called oxide layer.
Of most relevance to the present invention is commonly assigned U.S. Pat. No. 5,536,591 ("the '591 patent") in which the oxide (metal/electrolyte) interface is addressed in detail and where teachings on composition, size and distribution of catalytic sites within the oxide interface was first provided.
The '591 patent taught that hydrogen storage and other electrochemical characteristics of the electrode materials thereof could be controllably altered depending on the type and quantity of host matrix material and modifier elements selected for making the negative electrode materials. The negative electrode alloys of the '591 patent were resistant to degradation by poisoning due to the increased number of selectively designed storage and catalytically active sites which also contributed to long cycle life. Also, some of the sites designed into the material could bond with and resist poisoning without affecting the active hydrogen sites. The materials thus formed had a very low self discharge and hence good shelf life.
As discussed in U.S. Pat. No. 4,716,088 ("the '088 patent"), the contents of which are specifically incorporated by reference, it is known that the steady state surface composition of V--Ti--Zr--Ni alloys can be characterized as having porous, catalytic regions of enriched nickel. An aspect of the '591 patent was a significant increase in the frequency of occurrence of these nickel regions as well as a more pronounced localization of these regions. More specifically, the materials of the '591 patent had discrete nickel regions of 50-70 Angstroms in diameter distributed throughout the oxide interface and varying in proximity from 2-300 Angstroms or preferably 50-100 Angstroms, from region to region. This was illustrated in the FIG. 1 or the '591 patent, where the nickel regions 1 were shown as what appear as particles on the surface of the oxide interface 2 at 178,000.times.. As a result of the increase in the frequency of occurrence of these nickel regions, the materials of the '591 patent exhibited significantly increased catalysis and conductivity.
The increased density of Ni regions in the materials of the '591 patent provided metal hydride powder particles having a highly catalytic surface. Prior to the '591 patent, Ni enrichment was attempted unsuccessfully using microencapsulation. The method of Ni encapsulation results in the expensive physical, chemical or electrochemical deposition of a layer of Ni at the metal-electrolyte interface. The deposition of an entire layer was expensive, excessive and resulted in no improvement of performance characteristics since this kind of encapsulated layer did not result in the production of the localized, finely distributed nickel regions of 50-70 Angstrom in a porous matrix.
The enriched Ni regions of the '591 patent could be produced via two general fabrication strategies. The first of these strategies was to specifically formulate an alloy having a surface region that is preferentially corroded during activation to produce the described enriched Ni regions. It was believed that Ni was in association with an element such as Al at specific surface regions and that this element corroded preferentially during activation, leaving the enriched Ni regions described in the '591 patent. "Activation" as used herein specifically refers to "etching" or other methods of removing excessive oxides, such as described in the '088 patent as applied to electrode alloy powder, the finished electrode, or at any point in between in order to improve the hydrogen transfer rate.
The second of these strategies was to mechanically alloy a secondary alloy to a hydride battery alloy, where the secondary alloy preferentially corroded to leave enriched nickel regions. An example of such a secondary alloy was given as NiAl. The most preferred alloys having enriched Ni regions were alloys having the following composition: (Base Alloy).sub.a Co.sub.b Mn.sub.c Fe.sub.d Sn.sub.e ; where the Base Alloy comprised 0.1 to 60 atomic percent Ti, 0.1 to 40 atomic percent Zr, 0 to 60 atomic percent V, 0.1 to 57 atomic percent Ni, and 0 to 56 atomic percent Cr; b was 0 to 7.5 atomic percent; c was 13 to 17 atomic percent; d was 0 to 3.5 atomic percent; e was 0 to 1.5 atomic percent; and a+b+c+d+e=100 atomic percent.
The production of the Ni regions of the '591 patent was consistent with a relatively high rate of removal through precipitation of the oxides of titanium and zirconium from the surface and a much lower rate of nickel removal, providing a degree of porosity to the surface. The resultant surface had a higher concentration of nickel than would be expected from the bulk composition of the negative hydrogen storage electrode. Nickel in the metallic state is electrically conductive and catalytic, imparting these properties to the surface. As a result, the surface of the negative hydrogen storage electrode was more catalytic and conductive than if the surface contained a higher concentration of insulating oxides. Many of the alloys of the '591 patent include Mn. The effects of the addition of Mn to these alloys was generally discussed in U.S. Pat. No. 5,096,667, the disclosure of which is incorporated herein by reference. The addition of Mn usually results in improved charging efficiency. This effect appears to result from the ability of Mn to improve the charging efficiency of alloys into which it is added by improving oxidation resistance and oxygen recombination. It has been observed that oxygen gas generated at the nickel hydroxide positive electrode recombined at the surface of the metal hydride electrode. Oxygen recombination is an especially aggressive oxidizer of its environment, even compared to the alkaline electrolyte.
It is possible that the modifier elements added to the Base Alloy of the '591 patent, particularly Mn and Fe, and most particularly Co, either alone, or in combination with Mn and/or Al for example, act to catalyze oxygen reduction, thereby avoiding or reducing the oxidation of the surrounding elements in the metal hydride alloy. It is believed that this function of the modified alloys reduces or even eliminates the formation and build up of detrimental surface oxide, thereby providing a thinner and more stable surface.
It is believed that several additional factors may explain the unexpected behavior of Mn and Fe in the Base Alloys of the present invention: (1) The combination of Mn and Fe may affect the bulk alloy by inhibiting the bulk diffusion rate of hydrogen within the metal through the formation of complex phase structures, either by effecting the grain boundaries or by affecting the equilibrium bond strength of hydrogen within the metal. In other words, the temperature dependence of the hydrogen bond strength may be increased thereby decreasing the available voltage and capacity available under low temperature discharge. (2) It is believed that the combination of Mn and Fe may result in a lower electrode surface area for metallurgical reasons by increasing the ductility of the alloy and thereby reducing the amount of surface area formation during the activation process. (3) It is believed that the combination of Mn and Fe to these alloys may inhibit low temperature discharge through the alteration of the oxide layer itself with respect to conductivity, porosity, thickness, and/or catalytic activity. The oxide layer is an important factor in the discharge reaction and promotes the reaction of hydrogen from the Base Alloy of the present invention and hydroxyl ion from the electrolyte. It is believed that this reaction is promoted by a thin, conductive, porous oxide having some catalytic activity.
The combination of Mn and Fe does not appear to be a problem under room temperature discharge, but has shown a surprising tendency to retard the low temperature reaction. The formation of a complex oxide could result in a subtle change in oxide structure such as pore size distribution or porosity. Since the discharge reaction produces water at the metal hydride surface and within the oxide itself, a small pore size may be causing a slow diffusion of K.sup.+ and OH.sup.- ions from the bulk of the electrolyte to the oxide. Under room temperature discharge where polarization is almost entirely ohmic to low temperature discharge where activation and concentration polarization components dominate the physical structure of the oxides with Fe and Mn compared to Mn alone could be substantially different.
Still another possible explanation is that Mn and Fe have multivalent oxidation states. Some elements within the oxide may in fact change oxidation state during normal state of charge variance as a function of the rate of discharge and can be both temperature, fabrication, and compositionally dependant. It is possible these multiple oxidation states have different catalytic activity as well as different densities that together effect oxide porosity. A possible problem with a complex oxide containing both Mn and Fe could be that the Fe component retards the ability of the Mn to change oxidation state if present in large quantities.
Throughout the preceding discussion with respect to the oxide it should be noted that the oxide also contains other components of the Base Alloy, such as V, Ti, Zr, Ni, and/or Cr and other modifier elements. The discussion of a complex oxide of Mn and Fe is merely for the sake of brevity and one skilled in the art should not infer that the actual mechanism cannot also include a different or more complex explanation involving other such elements.